An example of a prior art image data format conversion apparatus is described in Japanese Published Patent Application No. Hei. 8-242427 which has been filed by the applicant of the present invention. Hereinafter the image data format conversion apparatus will be described using FIG. 13.
In FIG. 13, reference numeral 101 designates a display control unit outputting image data (R, G, B), numeral 3 designates a converter receiving the image data (R, G, B) and converting the data into signals in YUV format, numeral 4 designates a vertical filter unit, numeral 5 designates a horizontal filter unit, numeral 8 designates a display unit synthesizing the respective outputs from the vertical filter unit 4 and the horizontal filter unit 5, numeral 401 designates a first vertical filter buffer storing a line of data output from the converter 3, numeral 402 designates a second vertical filter buffer storing a line of data output from the first vertical filter buffer 401.
The display control device 101 horizontally scans the image data (RGB) stored in an image memory in accordance with a display rate of the display unit 8 to output the data. However, since an RGB-only monitor is required as a display unit 8, the image data is converted from the RGB format to the YUV format employed for television by the converter 3.
When image data processing is performed in RGB format, the three elements, i.e., red, green, and blue, have to be subjected to the same processing. On the other hand, in the case of YUV format, it is possible to process only luminance Y among three elements of luminance Y, color-difference U, color-difference V, or the data process can be carried out by sampling the color-difference U and the color-difference V, utilizing a visual characteristic unique to human beings of being dull at changes of color information. This format allows a reduction in cost of a hardware to be integrated.
Next, in order to reduce flickers caused by interlace display specific to television, the vertical filter unit 4 cuts off high frequency components using a low-pass filter to reduce a difference in brightness between adjacent lines. Flickers are attributed to a characteristic unique to human beings that blinks of dark light are perceived at 30 Hz and blinks of bright light are perceived at 60 Hz or less. When an image having a large difference in brightness between adjacent lines, such as a computer graphic, is displayed on an interlace display unit, as interlace scanning is performed in the interlace display unit, light flashes on and off at 30 Hz on the display unit and the blinks are perceived as flickers.
Therefore, as the vertical filter unit 4 has three taps, that is, the filter unit 4 uses a low-pass filter for three dots of image data (three pixels) aligned in the vertical direction, three lines of data aligned in the horizontal direction are required. The first vertical filter buffer 401 and the second vertical filter buffer 402 store two lines of data aligned in the horizontal direction. The two lines of data and a next line of data are process targets for the vertical filter unit 4. Since the next line of data is input to the first vertical filter buffer 401 as soon as the data in the first vertical filter buffer 401 is transmitted to the second vertical filter buffer 402, the latest line data is always reserved. As described above, the low-pass filter is used for three dots of image data aligned vertically and each shifted from adjacent dots by one line. This prior art reduces hardware cost by filtering the luminance component Y only among the elements YUV.
While using a low-pass filter for the vertical direction, the horizontal filter unit 5 uses a low-pass filter for the horizontal direction. As a display unit like a TV has insufficient resolution of color information of adjacent pixels, inconsistency in color occurs when displaying an image of higher resolution on such display unit. Therefore, among the elements YUV, only the color-difference components U and V are horizontally filtered. In this case, unlike the case of the vertical filtering, as the horizontally-scanned data alone is processed, buffers such as the vertical buffers 401 and 402 are unnecessary.
Finally, the luminance component Y output from the vertical filter unit 4 and the color-difference components U and V output from the horizontal filter unit 5 are output together to the display unit 8, whereby conversion to a clear image with no flicker is realized.
Thus, the above configuration can implement an image data converting function which fits to characteristics of a display unit.
The prior art image data format conversion apparatus has the above-described configuration and can perform overlay of one image on another as long as the images have the same pixel ratio. However, it is impossible for the above configuration to perform overlay of images with different pixel ratios, for example, Video Graphics Array (VGA) image and Moving Pictures Experts Group (MPEG) image. Moreover, when reducing/expanding an original image to an arbitrary size to adapt the image to a pixel ratio of a screen on a display unit, another filter and buffer for image interpolating are required, causing problems such as high hardware cost.